1. Field of the Invention
The present invention relates to high energy laser systems, to beam delivery systems, and to laser peening systems suitable for use with stationary targets.
2. Description of Related Art
The use of mechanical shocks to form metals and to improve their surface properties has been realized for ages. In current industrial practice, a peening treatment of metal surfaces is accomplished by using high velocity shot. Treatment improves surface properties and, very importantly for many applications, results in a part displaying significantly improved resistance to fatigue and corrosion failure. A wide range of work pieces are shot peened in the aerospace and automotive industries. However, for many applications, shot peening does not provide sufficiently intense or deep treatment, or cannot be used because of its detrimental effect on the surface finish.
With the invention of the laser, it was rapidly recognized that the intense shocks required for peening could be achieved by means of a laser-driven, tamped plasma. B. P. Fairand, et al., “Laser Shot Induced Microstructural and Mechanical Property Changes in 7075 Aluminum,” Journal of Applied Physics, Vol. 43, No. 9, p. 3893, September 1972. Typically, a plasma shock of 10 kB to 30 kB is generated at metal surfaces by means of high energy density (about 200 j/cm2), short pulse length (about 30 nanoseconds) lasers. A thin layer of metal tape, black paint or other absorbing material on the metal surface provides an absorber to prevent ablation of the metal. A confining or tamping material such as water covers the surface layer providing an increased intensity shock. These shocks have been shown to impart compressive stresses, deeper and more intense, than standard shot peening. In testing, this treatment has been shown to be superior for strengthening work pieces from fatigue and corrosion failure. Laser peening is also used for forming and texturing surfaces.
One laser system which has been utilized for this purpose is described in our prior U.S. Pat. No. 5,239,408, entitled HIGH POWER, HIGH BEAM QUALITY REGENERATIVE AMPLIFIER. The laser system described in the just cited '408 patent comprises a high power amplifier in a master oscillator/power amplifier MOPA configuration capable of producing output pulses greater than 20 Joules per pulse with the pulse width on the order of 10 to 30 nanoseconds or less using a wavefront correcting configuration based on a stimulated Brillouin scattering SBS phase conjugator/mirror system.
In current commercial high energy laser peening processes, the laser beam position is held at a fixed location. The work piece being treated is moved through the laser beam to create the applied spot pattern while maintaining the desired incidence angles, spot sizes, and orientations. This requires automation and work piece holding fixtures to grip the work piece and move it through its programmed positions. This method becomes both costly and highly work piece specific, requiring considerable engineering to develop processes for new work pieces. In addition, work piece size is limited to the lifting and handling capacity of the automation utilized. Work pieces and structures larger than automation handling capacity (for example, >1 m and/or >100 kg) cannot be laser peened by the conventional work piece moving approach.
Flexible beam delivery systems are often based on the use of optical fibers. However, even at wavelengths where glass fiber transmission is normally high, the very high pulse energy and high peak power used in laser peening can damage the fibers and render them ineffective. For example, a 25J pulse is 100 times the maximum pulse energy (250 mJ) that can be delivered through a 1 mm multi-mode fiber. For single frequency beams, such as used in representative laser peening applications, glass fibers have even lower damage thresholds.
Approaches to actively scanning a laser beam for the delivery of high power carbon dioxide (CO2) cutting and welding lasers have been developed. Because of its 10 μm wavelength in the far infrared, the output of a CO2 laser cannot be delivered by glass fibers. Commercial articulated arms have been developed for high power CO2 lasers that consist of a series of hollow tubes connected by seven articulation points (commonly called knuckles), each of which houses a 45 degree mirror. There are a number of important drawbacks to an articulated arm for laser peening that lead us to develop an alternative approach:
1. Beam rotation—For laser peening, it is desirable to use a square beam (unlike CO2 lasers) and the out-of-plane reflection at each articulated joint would cause some degree of beam rotation. Although this could be compensated by appropriately rotating the square beam before it enters the arm, the precise orientation of each arm segment would need to be known. Since there are multiple arm positions for a given delivery angle to the part, each of the seven rotational joints would need to be accurately encoded.
2. Pointing accuracy—The arms in the apertures needed to transmit a peening beam typically have a pointing accuracy of only 1 mrad, corresponding to up to 1 mm error in the positioning of a 3 mm spot, as used for example in laser peening.
3. Optical losses—A standard seven-knuckle articulated arm would require seven mirror reflections between the input and the output, introducing optical losses during beam delivery that reduce efficiency of the system.
4. Length limitations—Articulated arms have a fixed length, allowing limited flexibility as to placement with respect to the work piece. The maximum delivery length would also be limited by the weight and mechanical stiffness of tubular arm segments and the bearing loads at each joint.
5. Process development—Typically, the articulated arm is designed to be quite flexible; its design under-constrained so that multiple knuckle configurations are possible for a given treatment spot. However, it is still possible to damage the arm by forcing it through disallowed paths or by causing collisions with the process robot. For this reason, much of the complex robot path development already associated with the moving part process would still be needed.
It is desirable to provide a system that provides sufficient flexibility to be able to treat large work pieces and work pieces “in situ” at customer facilities, like aircraft parts at an aviation repair station or large oil drilling work pieces at a pipe yard.